feat: Add host-based assistant avatars (Neko/Mochi)

- Add avatar field to hosts.json config for Neko and Mochi
- Create HostContext to provide host config to components
- Display host avatar and name in chat messages instead of generic Claude
- Add user avatar and first name to user messages
- Include avatar in /api/hosts endpoint response
- Fix appendFileSync import for debug logging

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -1,6 +1,7 @@
 import { useState, useCallback, useEffect } from 'react';
 import { AuthProvider, useAuth } from './contexts/AuthContext';
 import { SessionProvider, useSessionManager } from './contexts/SessionContext';
+import { HostProvider } from './contexts/HostContext';
 import { Sidebar } from './components/Sidebar';
 import { TabBar } from './components/TabBar';
 import { ChatPanel } from './components/ChatPanel';
@@ -128,9 +129,11 @@ function AuthenticatedApp() {
   }
 
   return (
-    <SessionProvider>
-      <AppContent />
-    </SessionProvider>
+    <HostProvider>
+      <SessionProvider>
+        <AppContent />
+      </SessionProvider>
+    </HostProvider>
   );
 }